Mystery of Ancient Extinction in Earth's Oceans Revealed

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About 375 million years ago, the diversity of species in the
Earth's oceans plummeted — not because more species were going
extinct, but because fewer new groups of organisms were forming.
A new study identifies a culprit: invasive species.

The crisis of the Late Devonian Period is typically considered to
be one of the "Big Five"
mass extinctions ; however, this terminology is inaccurate,
according to Alycia Stigall, the study researcher and an
associate professor of geology at Ohio University.

During a given point of time, some species are going extinct,
while others are being created, a process called speciation,
according to Stigall.

"During normal intervals, those numbers offset one another," she
said.

During a mass extinction, like the one at the end of the
Cretaceous Period
when the dinosaurs disappeared, the rate of extinction
exceeds the rate at which new species are created. Scientists
have known for some time that the Late Devonian was different,
according to Stigall.

During the Devonian Period, which began about 416 million years
ago, most life still lived in the seas, where armored fish, such
as the 4-ton
Dunkleosteus terrelli (thought to be
able to tear a shark in two) dominated.

Life on land was limited, but by the time the Devonian Period had
ended, the first seed plants, forests, insects and amphibians had
emerged. The crisis, however, primarily affected marine life.

Continents were also moving together, colliding and forming
mountain ranges, and behind them, depressions.
Sea levels were rising, and many inland oceans appeared. This
geographic change allowed some species to spread and establish
themselves in new environments, but their presence prevented
native species from diversifying.

To better understand what had happened with speciation rates,
Stigall created detailed evolutionary trees for four groups of
organisms: clams, a type of predatory armored shrimp and two
types of brachiopods, which are shelled, filter-feeding
organisms. (Unlike a clam, the two shells of a brachiopod are
different in size, and brachiopods use a specialized structure
called a lophophore instead of gills to feed and breathe.)

These family trees, which incorporated about 130 species, allowed
her to tell when and where new species emerged, as well as when
they went extinct. For example, one species of deep-water
brachiopod, called Schizophoria impressa, evolved in
what is now the New York state area, then with the next major
rise in sea level, the same species appeared in the Iowa basin,
and with the subsequent rise, it invaded the New Mexico basin.

Most new species arise because of a process called vicariance.
This occurs when a geographic barrier such as a mountain range or
river divides a population, and the subpopulations evolve into
new species. However, during the Late Devonian many invader
species gained access to new habitats, and their presence
prevented new species from forming, regardless of the barriers
present. This process is likely happening now, to Stigall.

"We have both a potential problem with speciation and a clearly
documented problem with extinction," Stigall said.

In fact, the Earth is believed to be in the midst of a
sixth major extinction event, thanks to human activities that
have fragmented natural habitat and caused many species to
disappear. Humans have also transported invasive species around
the world and created environments where invasive species that
can handle a variety of settings – think pigeons, raccoons and
rats – thrive.

Modern speciation rates are difficult to calculate because the
process takes 10,000 to 50,000 years, she said.

The study, which was funded by the National Science Foundation,
was published in the journal PLoS ONE on Dec. 29.